Steam generators are used both in fossil-fuel and nuclear power plants to generate working steam for driving turbines. Water is fed to heat-exchange surfaces connected directly or indirectly to the nuclear-reactor core in the case of a nuclear system; in the case of a fossil-fuel system, the surfaces are located in the furnace. A steam-water mixture is thus produced, and this mixture is sent to a separator that returns the water to the heat-exchange surfaces and sends the steam ultimately to the turbine. Since the design of both nuclear and fossil units are made with certain assumptions concerning the thermal energy present in the coolant being fed to the heat-exchange surfaces, it is desirable that the amount of steam "carried under" with the separated water be controlled. In general, the amount of steam carried under should be minimized.